Oh it was brought to my remembrance about this song I sang in primary school. I still remember it was the first song in the songbook. Haha..
Here is how it goes...

A smile is such a funny thing
It wrinkles up your face
And when it's gone
You'll never find
Its secret hiding place
He smiles at you
You smile at him
And so one smile makes two!

So smiling will result in an increase of ratio 2! (1, 2, 4, 8, 16, ...) Therefore it's in a geometric progression! Haha..